Indexing Metaphone?

Folks,

I'm building a database that does a lot of fuzzy-string searches.  As such, 
I'd like to index the metaphone results of certain columns.  However, I get 
this when I try:

jwnet=> CREATE INDEX idx_locations_metaphone ON 
locations(metaphone(location_name, 12));
ERROR:  parser: parse error at or near "12"
jwnet=>

is this a syntax error, or do I need to create a column to hold the metaphone 
values in order to index them?

-- 
-Josh Berkus